The present invention relates to a semiconductor device for switching power supply control, a startup circuit, and a startup method for a switching power supply device, and in particular relates to a semiconductor device for switching power supply control, a startup circuit, and a startup method for a switching power supply device, to supply power to a load connected to the secondary-side winding of a transformer.
In flyback-type switching power supply devices of the prior art, integrated circuit devices (hereafter “ICs”) for switching power supply control have been used which induce a voltage in the secondary side of a transformer by turning a switching element connected to the primary-side winding on and off. This IC for switching power supply control normally incorporates a startup circuit within a single-chip integrated circuit comprising a circuit configuration having numerous transistors and other components, and also employs a voltage-stabilizing capacitor, connected externally to an IC power supply terminal, to stabilize the power supply voltage which drives the IC for switching power supply control itself.
In a switching power supply device of this type, after input of the power supply on the transformer primary side, until the output voltage of the switching power supply device stabilizes, a startup current has been supplied to the capacitor from this startup circuit to charge the capacitor, and by supplying a feed voltage from the fully-charged capacitor to the IC power supply terminal to operate the IC, switching operation has been started. That is, the feed voltage of the IC for switching power supply control is, for example, easily obtained as an auxiliary output voltage after the completion of startup of the switching power supply device, but at least during startup a power supply must be supplied separately from the input voltage, and for this reason a startup circuit to start the IC has been necessary.
FIG. 9 is a block diagram showing an example of a switching power supply device. The switching power supply device has a bridge diode BD101, capacitors C101 to C103, diodes D101 and D102, a transformer T101, a power transistor PwT11, a resistor R101, a photocoupler PC101, a switching power supply control IC 100, and a voltage detection circuit 111. The switching power supply control IC 100 has a high-voltage input terminal VH, power supply terminal VCC, output terminal OUT, current detection terminal IS, ground terminal GND, feedback terminal FB, and other terminals, and also has a startup circuit 101 and pulse control portion 102 and similar. The switching power supply device shown in FIG. 9 is an example of a so-called insulated AC-DC converter in which a commercial 100 V AC power supply is rectified, and after passing through the transformer T101, prescribed power is supplied to the load 121.
The bridge diode BD101 rectifies the commercial 100 V AC power supply. The rectified DC voltage is applied to a series circuit in which the main winding N101 on the primary side of the transformer T101 and the power transistor PwT101, which is a switching element, are series-connected. The power transistor PwT101 is grounded via a resistor R101 for current detection, used to detect the current flowing therein.
The feedback terminal FB of the switching power supply control IC 100 is connected to the phototransistor PT101 of the photocoupler PC101. The current detection terminal IS is connected to the connection point of the power transistor PwT11 and the current detection resistor R101, and receives as input the voltage value detected by the resistor R101. The ground terminal GND is grounded. The output terminal OUT is connected to the gate of the power transistor PwT11. The power supply terminal VCC is connected to the auxiliary winding (hereafter called the coil) on the primary side of the transformer T101 via the diode D101.
In this way, the switching power supply control IC 100 operates by means of a voltage induced in the coil N103 after completion of startup of the switching power supply device. Here, a capacitor C102 to stabilize the voltage supplied from the coil N103 is externally connected to the power supply terminal VCC of the IC 100.
When starting up after input of the power supply, in the switching power supply control IC 100, a startup current is supplied to the capacitor C102 from the bridge diode BD101 via the startup circuit 101. Then, the capacitor C102 is charged by the startup current, and when the voltage at the power supply terminal VCC rises to a prescribed value higher than the voltage necessary for operation of the IC 100, the startup current from the startup circuit 101 stops.
The pulse control portion 102 of the IC 100 comprises an internal oscillation circuit. In the pulse control portion 102, a pulse-modulated pulse signal is output from the output terminal OUT corresponding to the output voltage level received by the feedback terminal FB (including information related to the load level) and to the voltage input to the current detection terminal IS, at a switching frequency determined by the oscillation frequency of the oscillation circuit, to execute on/off control of the power transistor PwT101.
The load 121 is connected, via a rectifying/smoothing circuit comprising the diode D102 and capacitor C103, to the main winding N102 on the secondary side of the transformer T101. Because a voltage detection circuit 111 is connected between the rectifying/smoothing circuit and the load 121, the voltage supplied to the load 121 can be detected. The voltage signal detected by the voltage detection circuit 111 is fed back to the feedback terminal FB of the IC 100 via the photodiode PD101 of the photocoupler PC101 and the phototransistor PT101. Because the voltage signal is transmitted via the photocoupler PC101 to the primary side as a feedback signal, the primary side and secondary side of the transformer T101 are electrically insulated.
FIG. 10 is a block diagram showing the configuration of a startup circuit of the prior art. The startup circuit 101 comprises a startup device 112, current amplification circuit 113, and switch circuit 114, and has a startup voltage input terminal 101a, startup current output terminal 101b, and control terminal 101c. Within the startup circuit 101, the startup voltage input terminal 101a is connected to the high-voltage input terminal VH of the IC 100, and the startup current output terminal 101b is connected to the power supply terminal VCC of the IC 100. The control terminal 101c is connected to the power supply voltage detection circuit 103.
A capacitor C102 is connected externally, via the power supply terminal VCC of the IC 100, to the startup current output terminal 101b of the startup circuit 101. The high voltage supplied to the primary side of the transformer T101 is input via the high-voltage input terminal VH. The startup device 112 is a high-breakdown voltage element, to which most of the high potential difference between the high-voltage input terminal VH and the power supply terminal VCC is applied, and has the function of protecting other elements from high voltages. The current amplification circuit 113 uses a current mirror which amplifies a constant current such that the startup current is constant, and outputs the startup current from the startup current output terminal 101b via the switch circuit 114 to charge the capacitor C102. The power supply voltage detection circuit 103 detects the voltage at the power supply terminal VCC, and outputs an on/off signal, which is a control signal of the startup circuit 101, to the switch circuit 114.
Here, an “on” signal is output to the switch circuit 114 to pass a startup current until the power supply voltage reaches the voltage necessary to operate the IC 100, and when the voltage reaches a prescribed value higher than the voltage enabling operation of the IC 100, an “off” signal is output to halt the startup current. When startup of the switching power supply device is completed in this way, switching operation of the switching power supply device starts, and a power supply voltage (Vcc) is input at a prescribed magnitude from the coil N103 of the transformer T101 to the startup current output terminal 101b of the startup circuit 101, via the power supply terminal VCC. The above-described prescribed value normally has two values, high and low, so as to impart hysteresis; when the power supply voltage (Vcc) is lower than the lower of the prescribed values, the startup circuit again begins operation.
FIG. 11 is a circuit diagram showing an example of the specific configuration of a startup circuit of the prior art. In the startup circuit 101, the startup device 112 comprises N-channel high-voltage junction field effect transistors J11 and J12 (hereafter simply called transistors J11 and J12). The gates of these transistors J11 and J12 are connected to ground.
The current amplification circuit 113 shown in FIG. 10 comprises mutually current mirror-connected P-channel MOS transistors MP11, MP12 (hereafter called transistors MP11 and MP12) and a resistor R11. The switch circuit 114 comprises N-channel MOS transistors MN11, MN12 (hereafter called transistors MN11 and MN12) and resistors R12, R13.
The drains of the transistors J11 and J12 are both connected to the high-voltage input terminal VH, and the source of transistor J11 is connected to the sources of transistors MP11 and MP12. The source of transistor J12 is connected via resistor R13 to the drain of transistor MN11 and the gate of transistor MN12, and a pull-up voltage is applied to the gate of the transistor MN12. The gate of the transistor MN11 is connected to the control terminal 101c, so that an on/off signal serving as the control signal of the startup circuit 101 is input.
In such a startup circuit 101, when a high voltage is applied to the startup voltage input terminal 101a, the larger the potential difference across the source and gate of the transistor J11, the smaller is the drain current flowing from the startup device 112. In other words, the drain current decreases as the source potential of the transistor J11 increases. If the voltage drop across the transistor MP11 is neglected, then the current flowing in the transistor MP12 comprised by the current amplification circuit 113 is determined by a current which is determined by dividing the source potential of the transistor J11 by a resistance value of the resistor R11, and the current mirror ratio of the transistors MP11 and MP12.
FIG. 12 shows the voltage/current characteristics of a general junction field effect transistor. The curve IJFET shown in FIG. 12 shows the relation between the source-gate voltage (Vsg, [V]) and the drain current (Idr, [A]) of a junction field effect transistor.
As shown in FIG. 12, the larger the source-gate potential difference Vsg of the transistor J11, shown along the horizontal axis, the more there is an exponential decrease in the drain current Idr, plotted along the vertical axis. Hence immediately after startup of the switching power supply device, the voltage at the power supply terminal VCC is near 0 V, so that the potential difference between gate and source of the transistor J11 is small, and a large current flows.
The straight line ICM shown in FIG. 12 is a characteristic of the current resulting from current-mirroring of the current flowing in the resistor R11 from the transistor MP11 to the transistor MP12 (as stated above, the voltage drop across the transistor MP11 is neglected). The voltage Vsg applied across the gate and source of the transistor J11 is also the voltage applied to the resistor R11 via the transistor MP11, and if the voltage drop due to the transistor MP11 is neglected, the current flowing in the resistor R11 is proportional to the voltage Vsg. That is, the current flowing in the transistor MP12 determined by the current mirror operation has the characteristic represented by the straight line ICM. Because the current flowing in the transistor J11 and the current flowing in the PMOS transistor MP12 are equal (however, because the mirror ratio is large, the current flowing in resistor R11 is neglected), the point of intersection of the curve IJFET and the straight line ICM is the stable point which is sought.
This stable point is determined by the voltage/current characteristic across source and gate of the transistor J11, the resistor R11, and the threshold value (Vth) of the transistor MP11; the source voltage of the transistor J11 is a constant-voltage value Vconst, unrelated to the voltage value of the power supply terminal VCC or the voltage value of the high-voltage input terminal VH. When a high voltage is applied to the startup voltage input terminal 101a shown in FIG. 11, if the resistor R11 is 3 MΩ and the source potential of the transistor J11 is Vconst=30 V, then the current flowing through the resistor R11 and transistor MP11 is a constant 10 μA current. The gate dimensional ratio W/L of the transistors MP11 and MP12 is assumed to be 1:100, and the startup current flowing from the transistor J11 to the startup current output terminal 101b is a constant 1 mA current.
At the time of startup of the switching power supply IC 100, a low-voltage malfunction prevention circuit such as the power supply voltage detection circuit 103 shown in FIG. 10 causes an L (low) state off signal to be input to the transistor MN11, turning off the transistor MN11. At this time, a high voltage is input to the gate terminal of the transistor MN12, so that the transistor MN12 is in the on state, and the startup circuit 101 operates so as to pass the drain current Idr of the transistor J11, and consequently a startup current begins to flow from the high-voltage input terminal VH toward the power supply terminal VCC.
When the voltage at the power supply terminal VCC rises to the above-described prescribed value (the higher value), an H (high) state on signal is output from the power supply voltage detection circuit 103, and the transistor MN11 comprised by the switch circuit 114 is turned on. Then, the gate potential of the transistor MN12 goes to the L state, the transistor MN12 enters the off state, and the supply of the startup current from the startup circuit 101 shown in FIG. 11 to the power supply terminal VCC stops.
FIG. 13 shows changes in the startup current in a startup circuit of the prior art. The horizontal axis in the figure shows the power supply voltage (Vcc, [V]) at the power supply terminal VCC, and the vertical axis indicates the startup current (Istup, [mA]) from the startup current output terminal 101b of the startup circuit 101.
Here, if the source potential at the above-described stable point is Vconst=18 V, then when the power supply voltage Vcc exceeds this 18 V, the source potential cannot be maintained at the 18 V of the stable point, but instead rises, so that the startup current Istup declines corresponding to the curve IJFET in FIG. 12. When the power supply voltage Vcc rises to the above-described prescribed value (higher value), the transistor MN12 enters the off state, and the startup current Istup becomes zero. If the resistor R11 does not have a temperature dependence (if the resistance value does not change with temperature), then as shown in the figure, a constant-current characteristic is maintained for the current value Istup at the power supply terminal VCC at least until the voltage at the power supply terminal VCC reaches 18 V. However, because this constant-current value has a temperature characteristic to some extent, the capacitor C2 connected externally to the power supply terminal VCC of the switching power supply control IC 100 is charged at a constant current determined by the temperature.
Japanese Patent Application Laid-open No. 2006-204082 describes an invention of a semiconductor device for switching power supply control in which, at the time of startup, the startup current passed from the startup device to the power supply terminal is made constant by a constant startup current circuit. Here, a capacitor connected externally to the power supply terminal is charged by a constant current, so that heat generation when the power supply terminal voltage is low is suppressed, and faults when the power supply terminal is shorted to ground are prevented; in addition, the power supply design is simplified.
In U.S. Pat. No. 6,940,320, and in Japanese 2007-509493 corresponding to U.S. Patent Application No. 2005/077551 A1, a power supply control system startup method is described in which two constant-current sources, large and small, are prepared for currents to be passed to the startup circuit; initially a small initial current value is used to raise the output to an initial voltage value, and when this has risen to a certain extent, the current source is switched to the large constant-current source to raise the voltage value up to the operating voltage value.
In the above-described switching power supply devices, at the time the power supply is input the startup circuit 101 receives a high voltage from the high-voltage input terminal VH, and generates a current sequence to charge the capacitor C102 for voltage stabilization. In this case, even if the power supply terminal VCC is shorted to ground, a configuration has been necessary to ensure that to the extent possible a large current does not flow, in order that the switching power supply control IC 100 does not generate heat nor is not destroyed due to combustion.
The technology of the prior art disclosed in Japanese Patent Application Laid-open No. 2006-204082 prevents faults when the power supply terminal VCC is shorted to ground by executing control such that the startup current is a constant magnitude during startup; but because a configuration is employed which passes a charging current with a constant value regardless of the voltage at the power supply terminal VCC, if the constant current value is too small, a long time is required until power supply startup. If the current value of the startup current is set to a large value in order to shorten the startup time, then there is the problem that heat generation and combustion cannot reliably be prevented when an anomaly occurs.
In the technology of the prior art described U.S. Pat. No. 6,940,320 and U.S. Patent Application No. 2005/0077551 A1, a configuration is employed in which the fact that the potential at the power supply terminal VCC has risen to an initial voltage value is detected, and switching is then performed to a circuit to supply a large startup current. For this reason, there is the advantage that when the power supply terminal VCC is shorted to ground, a large current does not flow.
However, a fault in a switching power supply device is not necessarily a fault in which the power supply terminal VCC is completely shorted to ground, and because Zener circuit elements are comprised, it is conceivable that after the voltage has risen to a certain extent a large current flows in the control circuit, and as a result a heat generation or combustion fault occurs. Hence in the technology of the prior art, there has been the problem that heat generation and combustion of the switching power supply control IC cannot be reliably prevented.